1. Field of the Invention
The present invention generally relates to a magnetic resonance imaging apparatus, and more particularly, to a sequence controller capable of controlling the simultaneous execution of several events such as the application of the magnetic field gradients to a body under examination in a magnetic resonance imaging apparatus.
2. Description of the Related Art
The magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Each such nucleus has a net magnetic moment such that when placed in a static homogeneous magnetic field, H0, a greater number of nuclei align with the H0 field to create a net magnetization, M, in the direction of the field. Net magnetization M is the summation of the individual nuclear magnetic moments. Because a nuclear magnetic moment is the result of a nuclear spin, the terms "nuclear moment" and "nuclear spin" as used herein are synonymous.
Under the influence of the magnetic field H0, the nuclei (and hence the net magnetization M) precess or rotate about the axis of the field. The rate (frequency) at which the nuclei precess is dependent on the strength of the applied magnetic field and on the nuclei characteristics. The angular frequency of precession, .omega., is defined as the Larmor frequency and is given by the equation EQU .omega.=.gamma.H0 (1)
in which .gamma. is the gyromagnetic ratio (constant for each type of nucleus) and H0 is the strength of the applied static homogeneous magnetic field. The frequency at which the nuclei precess is thus primarily dependent on the strength of the magnetic field H0 and increased with increasing field strength.
A precessing nucleus is capable of absorbing electromagnetic energy. The frequency of the electromagnetic energy needed to induce resonance is the same as the precession frequency .omega.. During the application of the electromagnetic energy, typically a radio frequency (RF) pulse, the net magnetization M precesses further and further away from the z-axis (arbitrarily assumed to be the direction of the H0 field), depending on the energy and duration of the RF pulse. A 90.degree. RF pulse causes the magnetization M to depart 90.degree. from the direction of the H0 field into the x-y plane defined by the x- and y-axis, for example, of the Cartesian coordinate system. Similarly, a 180.degree. RF pulse causes the magnetization M to reverse direction by 180.degree. from its original direction (from the positive z-axis direction to negative z-axis direction, for example). Following the excitation of the nuclei with RF energy, the absorbed energy is reradiated as an NMR signal as the nuclei return to equilibrium. The energy is emitted as radio waves and also transferred to surrounding molecules.
The above-described magnetic resonance imaging system is known in the art, for instance, from U.S. Pat. No. 4,471,306 to Edelstein et al. issued on Sept. 11, 1984, and U.S. Pat. No. 4,318,043 to Crooks et al. issued on Mar, 2, 1982.
In a conventional magnetic resonance imaging apparatus of this type, a sequence controlling circuit, a so-called "pulse sequencer", is arranged in order to realize a series of sequence control operations as above. In this case, since the sequence control is not always fixed but varies in accordance with imaging methods, the pulse sequencer must be programmable. Therefore, a microcomputer or microprocessor is conventionally used for the pulse sequencer.
However, the processing speed of the microcomputer is not high enough to execute sequence control necessary for high-speed imaging that will be developed in future. Events such as generation of gradient fields are often preferably executed simultaneously in an imaging process. In the conventional pulse sequencer using the microcomputer, a plurality of events cannot be actually executed simultaneously. More specifically, even when simultaneous execution of a plurality of events is desired, a slight time shift must be allowed.
In the sequence controlling circuit such as the pulse sequencer, which is controlled by software of the microcomputer in the conventional magnetic resonance imaging apparatus, high-speed control is inherently hard to attain, and it is also difficult to simultaneously execute a plurality of events.
The present invention has been made in order to solve the conventional problems, and has as its object to provide a magnetic resonance imaging apparatus including a sequence controlling circuit capable of high-speed sequence control and simultaneous execution of a plurality of events.
Another object of the present invention is to provide a magnetic resonance imaging apparatus which can realize a sequence control system by means of hardware, instead of a sequence control system by means of software.